As presented in FIG. 1, the important parts of a human eye 100 include the cornea 110, the iris 120, the pupil 130, the lens 135, the retina 140, and the optic nerve 150. The cornea 110 is the clear outer bulging surface at the front of the eye. The cornea 110 is the major refractive surface of the eye. The iris 120 is the colored part of the eye that regulates an amount of light entering the eye. The pupil 130 is the opening at the center of the iris 120. The iris 120 modifies or adjusts the size of the pupil 130 and thereby controls the amount of light that enters the eye. The lens 135 is the clear part of the eye behind the iris 120 that focuses light on the retina 140. The retina 140 is a light-sensitive tissue lining at the back of the eye. The retina 140 transforms light into electrical signals that are transmitted to the brain via the optic nerve. The aqueous humor 160 is a gelatinous fluid that is located in the space between the lens 135 and the cornea 110. The function of the aqueous humor 160 is to maintain the intraocular pressure and inflate the globe of the eye.
Gaze tracking is the process of tracking the point of gaze (the line of sight associated with an eye or what an eye is looking at) over a period of time. At a basic level, gaze tracking includes the steps of illuminating the eye using a light source, thereby causing visible reflections from various boundaries of the eye. Some of these reflections may be referred to as Purkinje images. One type of visible reflection that is tracked is the glint. The glint is the small amount of light that is reflected by the cornea. The glint may also be referred as the first Purkinje image.
As used herein, gaze tracking may include the steps of generating the reflection and tracking the reflection. The step of generating the reflection may be performed using discrete infrared (IR) light sources, e.g., discrete IR light-emitting diodes (LEDs) that are used to direct IR light into an eye. There are several advantages associated with using IR light for gaze tracking. IR light can illuminate an eye without disturbing a viewer. Additionally, IR light is reflected well by the cornea or by other parts of the eye (e.g., the pupil), and consequently, the reflections are more easily detectable by an IR image-capturing module (e.g., an IR camera).
However, there are several disadvantages associated with using discrete IR LEDs to direct light into an eye. In systems where the distance from the image-capturing module to the eye is very small, reflections (e.g., the glint) may be difficult to detect and/or track with high precision as the reflections become larger. Additionally, an IR LED can occupy a substantial amount of space, and since space on a gaze tracking system may be limited, the system cannot accommodate several IR LEDs when several IR LEDs are required. Additionally, when a system needs to accommodate several IR LEDs, each LED needs to be separated by at least a predetermined distance in order to enable the reflection (e.g., the glint) to fall within the spherical part of the cornea. When the space on a system is limited, the system cannot accommodate several LEDs because the limited space prevents each LED from being separated from another LED by the predetermined distance. Additionally, the angular range over which the direction of a gaze can be tracked is small because eye movement causes the reflection (e.g., the glint) to fall outside the spherical part of the cornea, which makes the detection of the reflection less reliable.
Therefore, what is needed is a system for gaze tracking that overcomes these disadvantages.